This proposal is aimed at continuing our studies of a novel TCR-coupled signaling protein, SWAP-70-like adapter of T cells (SLAT), which is predominantly expressed in T cells. SLAT is a Cdc42/Rac1 GEF that is required for inflammatory responses mediated by Th1, Th2 and Th17 cells, reflecting its obligatory role in TCR-stimulated Ca2+ release from intracellular ER stores and, consequently, in NFAT (but not NF-:B or AP-1) activation. Our work also revealed that antigen-induced translocation of SLAT to the immunological synapse (IS) is mediated by Lck kinase-dependent phosphorylation of a SLAT ITAM-like motif, and that the Ca2+/NFAT regulatory activity of SLAT depends on actin polymerization and active Cdc42/Rac1. The overall objective of the proposed studies is to further understand the function and regulation of SLAT in T cell biology, to determine its role in immunity to infectious pathogens, and to validate it as a promising, T cell-specific immunosuppressive drug target. Our studies will address the following Aims: Aim 1) We will use high-resolution imaging approaches to determine the spatiotemporal pattern of wild-type SLAT and SLAT mutants vis--vis the IS and TCR microclusters (MCs), and determine how Slat deletion affects IS dynamics; Aim 2) Mass spectrometry (MS) analysis of proteins binding the phospho-ITAM motif of SLAT and co-immunoprecipitation experiments identified the ERM protein ezrin as a SLAT-interacting protein. We will use biochemical and genetic approaches to address the hypothesis that the interaction of SLAT with ezrin plays an important role in controlling IS dynamics and couples SLAT to Cdc42/Rac1 activation; Aim 3) Based on our preliminary finding of a TCR-induced association between SLAT and the inositol 1,4,5-trisphosphate receptor type 1 (IP3R1), we will collaborate with experts in the IP3R field to analyze the structural determinants of this interaction and its functional relevance for T cell activation, and address the working hypothesis that SLAT is required for proper IP3R function in T cells; Aim 4) SLAT is required for immune/inflammatory responses against non-replicating antigens, but its role in pathogen clearance, in which CD8+ T cells are critical, is unknown. We will build upon our preliminary findings documenting a role for SLAT in CD8+ T cell activation and expansion to study whether SLAT is required for pathogen (Listeria Monocytogenes and Vaccinia virus) clearance, and whether the Ca2+ signaling defect is the cause of impaired CD8+ T cell responses observed in Slat-/- mice. Results of the proposed research will represent significant advancement in both the basic/scientific and clinical arenas. First, they will shed light on the mechanistic aspects of a key novel player in the TCR-induced Ca2+/NFAT signaling pathway, which plays a critical role in various aspects of T cell biology. Second, they are likely to validate SLAT as a novel drug target for autoimmune diseases and in doing so serve as a launching point for identification of clinically useful, T cell- and Ca2+ signaling pathway-selective drugs.